By 1813 the revolution in Buenos Aires had reasons for cautious hope - victories at Tucumán and Salta had steadied a shaky cause. But the royalist stronghold of Montevideo, though under siege, still controlled the river, and its ships ranged up the Paraná raiding coastal towns for food and money. The new government promoted San Martín to colonel and gave him the Regiment of Mounted Grenadiers, a unit he had trained in the latest tactics of the Napoleonic Wars, and told him to stop the raids. When he learned the royalists meant to plunder the San Carlos Convent near the village of San Lorenzo, he pushed his men hard to get there first.

The plain offered no cover - which was the problem, and also, in the end, the solution. The cavalry could not hide in open country, so on the night of February 2 the grenadiers slipped in through the rear door of the San Carlos Convent and went silent. No fires. No talking. From the convent tower, San Martín studied the enemy ships and the ground below through a monocular, working out exactly where the royalists would land and how he would catch them. The reinforcements mattered: a cadet named Ángel Pacheco had ridden ahead and staged fresh horses along the route, which is why an army covered the distance from Buenos Aires in only five days.

The royalists came ashore at 5:30 in the morning, formed two columns with their cannon, and started up the ladder-shaped path cut into the bank. San Martín gave a short speech, divided his roughly 120 horsemen into two wings, and sent them in - one led by him, the other by Justo Germán Bermúdez. The grenadiers carried no firearms into the charge; they fought with sabre and lance. Surprise and speed undid a force nearly twice their size. The royalists could not form a defensive square, and they broke and fled back toward their ships under covering fire. In a quarter of an hour it was decided. Forty royalist soldiers lay dead. Fourteen grenadiers were killed outright, and two more would die of their wounds.

San Martín nearly died in the dust. His horse was shot from under him and fell, pinning his leg, and a royalist - probably the enemy commander himself - slashed his face and put a bullet through his arm. Two of his own men reached him in time: Juan Bautista Baigorria and Juan Bautista Cabral. Cabral was mortally wounded pulling his commander free. San Martín later wrote that Cabral, dying, said he was content - that they had beaten the enemy. Cabral is remembered across Argentina as Sergeant Cabral, though he was in fact a private; some say San Martín, who also spoke Guaraní, translated the young soldier's last words from his first language. Bermúdez, who led the other wing, was shot in the knee and died within days. They are not statistics. The monument here keeps nine memorials for the sixteen dead, grouped by the nine homelands they came from: six Argentine provinces, plus Chile, France, and the Banda Oriental that would become Uruguay.

What San Martín did next is remembered almost as much as the fight. He took no hostages and demanded no ransom. When the defeated commander, Zabala, asked for help tending his wounded, San Martín gave it - and then invited Zabala to share a large breakfast, hoping to persuade him that the cause of absolute monarchy was not worth dying for. He succeeded; Zabala later joined the patriot army. The battle's military importance was slight, and it changed little in the wider war. But it became legend because of who fought it, and how. The march written for it is counted among the finest in the world, and the stone pine under which San Martín wrote his battle report - the Historic Pine - still stands beside the convent, more than two centuries old.